If memory serves the dell psu has an odd pin-out on it, different connectors. Maybe not, but check it out. You may have to get the psu from dell, which will kill your budget. Also, make sure that the video card you get is compatible with the bus that is in the dell - but you probably thought of that.
If you want to do this upgrade for fun and learning, go ahead. If you have a limited budget, cost the parts you need first, make sure they are compatible, and you will probably find that your local computer shop can give you a new computer for less. Save the dell as it is, put xfce on it or some lightweight linux distribution, and make it a media player machine.
My two cents. Welcome to LQ !